struct Foo: Codable {
    
    struct Value<T: Codable>: Codable {
        var value: T
        
        init(default def: T) {
            value = def
        }
        
        init(from decoder: Decoder) throws {
            let container = try! decoder.singleValueContainer()
            value = try! container.decode(T.self)
        }
        
        func encode(to encoder: Encoder) throws {
            var container = encoder.singleValueContainer()
            try! container.encode(value)
        }
    }   
    
    var test = Value<String?>(default: nil)
}



var f1 = Foo()
let data = try! JSONEncoder().encode(f1)
print(String(data: data, encoding: .utf8)!) // {"test":null}

let f2 = try! JSONDecoder().decode(Foo.self, from: data)

Fatal error: 'try!' expression unexpectedly raised an error: Swift.DecodingError.valueNotFound(Swift.Optional<Swift.String>, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "test", intValue: nil)], debugDescription: "Expected Optional<String> but found null value instead.", underlyingError: nil)): file /BuildRoot/Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-902.0.48/src/swift/stdlib/public/core/ErrorType.swift, line 184